This invention relates to a method for regeneration of a supported ruthenium catalyst which has been used for the conversion of a carbohydrate to a polyhydric alcohol by reaction with hydrogen.
The term "carbohydrate" as used throughout the specification and claims includes monosaccharides and polysaccharides. This term includes both pure compounds, such as glucose and sucrose, and mixtures such as cornstarch hydrolyzate, which is a hydrolysis product of cornstarch containing glucose (dextrose) and oligomers thereof.
Ther term "polysaccharide" as used in the specification and claims includes those saccharides containing more than one monosaccharide unit. This term encompasses disaccharides and other saccharides containing a small number of monosaccharide units, which are commonly known as oligosaccharides.
The term "conversion" as used herein refers to hydrogenation when applied to monosaccharides and to a combination of hydrogenation and hydrolysis when applied to polysaccharides.
Our copending parent application Ser. No. 520,926 now U.S. Pat. No. 3,963,788 describes and claims a process for the conversion of a carbohydrate to a polyhydric alcohol using a ruthenium-containing zeolite having a silica/alumina mol ratio greater than 3, particularly a ruthenium-containing Y type zeolite, as the catalyst. The ruthenium is present as the free metal on the zeolite which serves as a support. Glucose and cornstarch hydrolyzate, both of which yield sorbitol, are representative carbohydrates. Our copending parent application Ser. No. 531,972 now U.S. Pat. No. 3,963,789 describes and claims a process for conversion of a polysaccharide-containing carbohydrate, such as cornstarch hydrolyzate, to a polyhydric alcohol using ruthenium on a crystalline aluminosilicate clay as the catalyst. Both of these applications describe regeneration of the catalyst with a dilute aqueous mineral acid, as will be described and claimed herein.
Other processes for converting monosaccharides and polysaccharides to polyhydric alcohols using supported ruthenium catalysts were known prior to the inventions described in our above-cited copending applications. For example, U.S. Pat. No. 2,868,847 discloses the use of ruthenium on an inert catalyst support such as carbon, alumina, silica, or kieselguhr as a catalyst for the catalytic hydrogenation of saccharides such as dextrose, levulose, sucrose, maltose, and lactose. Starting materials include monosaccharides, e.g. dextrose and levulose, and disaccharides, e.g. sucrose, lactose, and maltose. Dextrose was hydrogenated to sorbitol and sucrose and lactose were hydrolyzed and hydrogenated to hexitols. However, maltose, a disaccharide containing two glucose units, was more easily converted to maltitol, a C.sub.12 alcohol, according to the patent.
U.S. Pat. No. 3,055,840 discloses the hydrogenation of various carbonyl compounds, including glucose (which yields sorbitol on hydrogenation), using a promoted ruthenium catalyst on a solid carrier. Various solid carriers including carbon, silica gel, alumina, kieselguhr, bentonite, and titanium dioxide, are disclosed.
The hydrogenation of monosaccharides using a supported ruthenium, palladium, platinum, or nickel catalyst (activated carbon was used as the support in all experimental work) is discussed in an article by N. A. Vasyunina et al., "Catalytic Properties of Ruthenium in Monosaccharides Hydrogenation Reaction", in Izvestiya Akademii Nauk SSR Khimicheskaya Seriya 4:848-854 (1969). Ruthenium was found to have higher activity than the other three catalysts.
A two stage process for hydrogenation of ligneous and other plant material such as wood sawdust is disclosed in Izv. Akad Nauk SSR, Otd. Khim. 8: 1522-1523 (1960). The process consists of a first stage hydrolytic hydrogenation of polysaccharides in an acid medium, followed by a second stage hydrogenation of the lignin in an alkaline medium, using a ruthenium catalyst in both stages. In a specific embodiment, pine sawdust is treated using an aqueous phosphoric acid medium and a ruthenium on carbon catalyst. The first stage reaction product is filtered to separate the liquid medium from the crystals obtained from the first stage filtrate.
None of the above references describes regeneration of the supported ruthenium catalyst.
Various nickel catalysts for conversion of carbohydrates to polyhydric alcohols are also known. U.S. Pat. Nos. 3,538,019 and 3,670,035 and the references cited therein are examples of such catalysts.
The supported nickel catalysts described in U.S. Pat. Nos. 3,538,019 and 3,670,035 (which is a division of U.S. Pat. No. 3,538,019) have high activity for the conversion of both monosaccharides and polysaccharides, including carbohydrate mixtures such as cornstarch hydrolyzate, with high selectivity to sorbitol when either cornstarch hydrolyzate or dextrose is used as the starting material. Carbon, diatomaceous earth, and kieselguhr are disclosed as carriers. This represents a significant improvement over the process and catalyst of U.S. Pat. No. 2,868,847, since the relatively inexpensive cornstarch hydrolyzate, or other commercially available carbohydrate mixtures, can be used as the starting material in place of the much more expensive pure sugars. A disadvantage of the catalyst in U.S. Pat. Nos. 3,538,019 and 3,670,035 is that the catalyst cannot be regenerated; when reactivation is required, it is necessary to remove the active catalyst material from the support by chemical means and then to redeposit the catalyst metal on the support. Various other nickel catalysts for conversion of carbohydrates to polyhydric alcohols are cited in U.S. Pat. Nos. 3,538,019 and 3,670,035.
Although various catalytic processes for the conversion of carbohydrates to polyhydric alcohols are known in the art, none possesses all of the attributes which are desirable in such processes, e.g., ability to use inexpensive mixed carbohydrates; high selectivity to sorbitol when either glucose or a starch hydrolyzate is used as the starting material; high catalyst attrition resistance; and ease of catalyst regenerability.